Ophthalmic perception, diagnostics and/or surgical procedures involve a variety of equipment such as frequency multiplied infrared lasers, solid state lasers, radio frequency energy sources and ultrasound systems, just to name a few. In each of these systems/procedures, knowledge and/or control of eye position and movement is critical.
For example, photorefractive keratectomy (PRK) is a procedure for laser correction of focusing deficiencies of the eye by modification of corneal curvature. PRK is distinct from the use of laser-based devices for more traditional ophthalmic surgical purposes, such as tissue cutting or thermal coagulation. PRK is generally accomplished by use of a 193 nanometer wavelength excimer laser beam that ablates away corneal tissue in a photo decomposition process. Most clinical work to this point has been done with a laser operating at a fluence level of 120-195 mJ/cm.sup.2 and a pulse-repetition rate of approximately 5-10 Hz. The procedure has been referred to as "corneal sculpting."
Before sculpting of the cornea takes place, the epithelium or outer layer of the cornea is mechanically removed to expose Bowman's membrane on the anterior surface of the stroma. At this point, laser ablation at Bowman's layer can begin. An excimer laser beam is preferred for this procedure. The beam may be variably masked during the ablation to remove corneal tissue to varying depths as necessary for recontouring the anterior stroma. Afterward, the epithelium rapidly regrows and resurfaces the contoured area, resulting in an optically correct (or much more nearly so) cornea. In some cases, a surface flap of the cornea is folded aside and the exposed surface of the cornea's stroma is ablated to the desired surface shape with the surface flap then being replaced.
Phototherapeutic keratectomy (PTK) is a procedure involving equipment functionally identical to the equipment required for PRK. The PTK procedure differs from PRK in that rather than reshaping the cornea, PTK uses the excimer laser to treat pathological superficial corneal dystrophies, which might otherwise require corneal transplants.
In both of these procedures, surgical errors due to eye position errors including both initial centration errors between the eye and the surgical laser and/or subsequent movement caused by involuntary (saccadic) eye movement, head movement or surgical equipment movement may degrade the refractive outcome of the surgery. The movement or positioning error is critical since the treatment laser's effectiveness depends on its being centered on the patient's theoretical visual axis which, practically speaking, is approximately the center of the patient's pupil. However, this visual axis is difficult to determine due in part to residual eye movement and involuntary eye movement known as saccadic eye movement. Saccadic eye movement is high-speed movement (i.e., of very short duration, 10-20 milliseconds, and typically up to 1.degree. of eye rotation) inherent in human vision and is used to provide dynamic scene images to the retina. Saccadic eye movement, while being small in amplitude, varies greatly from patient to patient due to psychological effects, body chemistry, surgical lighting conditions, etc.
One approach for dealing with eye position error is to try to eliminate it by using a grasping device or suction ring to physically hold the patient's eye stable during surgery. However, the intrusive nature of the suction ring may distort the eye's shape thereby affecting surgical precision. Further, since the suction ring is typically held by the surgeon, the surgeon's lower frequency but larger amplitude hand motions become a factor in surgical precision.
Another approach for dealing with eye position error is to non-intrusively sense this eye position. One sensing technique/system known in the prior art is the 1st and 4th Purkinje reflection tracker. The 1st and 4th Purkinje reflections refer to images based on the 1st Purkinje image which is the glint reflection off the front surface of the cornea and the 4th Purkinje image which is a reflection off the back of the eye's lens. This technique/system is used to track X-Y position of the eye. However, for certain corneal sculpting surgical procedures, the 1st Purkinje surface is ablated thereby rendering this technique/system ineffective for corneal sculpting.